String concatenation issue in C++

Question

I am trying to concatenate some string but it works in one but not another.

Working: I take in 2 argument and then do this. a = hello, b = world

string concat = a + b;

The output would be hello world with no problem.

Not working: I read from file and concatenate with 2nd argument. assuming string from file is abcdefg.

string concat = (string from file) + b;

and it gives me worldfg.

Instead of concatenating, string from b overwrites the initial string.

I have tried a few other methods such as using stringstream but it doesn't work as well.

This is my code.

int main (int nArgs, char *zArgs[]) {
    string a = string (zArgs [1]);
string b = string (zArgs [2]);
    string code;

    cout << "Enter code: ";
cin >> code;

    string concat = code + b;
}
// The output above gives me the correct concatenation.
// If I run in command prompt, and do this. ./main hello world
// then enters **good** after the prompt for code.
// The output would be **goodworld**

However, I read some lines from the file.

 string f3 = "temp.txt";
 string r;
 string temp;

 infile.open (f3.c_str ());

 while (getline (infile, r)) {
    // b is take from above
temp = r + b;
    cout << temp << endl;
 }
 // The above would give me the wrong concatenation.
 // Say the first line in temp.txt is **quickly**.
 // The output after reading the line and concatenating is **worldly**

Hope it gives more clear example.

Update:

I think I may have found out that the problem is due to the text file. I tried to create a new text file with some random lines inside, and it seem working fine. But if I try to read the original file, it gives me the wrong output. Still trying to put my head around this.

Then I tried to copied the content of the original file to the new file, and it seem to be working fine. Not too sure what is wrong here though. Will continue to test out, and hopefully it works fine.

Thanks for all the help! Appreciate it!

Answer 1

I get the same output as the chap who asked the original question:

$ ./a.out hello world
Enter code: good
goodworld
worldly

The problem here is the contents of the text file. For my example, the initial 7 characters in the text file are: "quickly". However, immediately following that are 7 backspace bytes (hex 08). This is what the contents looks like in emacs:

quickly^H^H^H^H^H^H^H

So how is this causing the mess?

Well the concatenation operation actually works correctly. If you do:

std::cout << "string length: " << temp.size() << "\n";

...you get the answer 19 which is made up of: "quickly" (7) + 7 backspace chars + "world"(5). The overwriting effect you observe is caused when you print this 19 char string to console: it is the console (eg xterm) that interprets the backspace sequence as meaning "move the cursor back to the left", thus removing earlier characters. If instead you pipe the output to file, you will see the full string (including the backspaces) is actually generated.

To get around this you might want to validate/correct the input that comes from the file. There are functions commonly available in C/C++ environments such as isprint(int c), iscntrl(int c) that you could make use of.

Update: as mentioned by another responder, other ASCII control characters will also have the same effect, eg, a carriage return (Hex 0D) will also move the cursor back to the left.

Answer 2

If I compile this

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main (int nArgs, char *zArgs[]) {
   string a = string (zArgs [1]);
   string b = string (zArgs [2]);
   string code;

   cout << "Enter code: ";
   cin >> code;

   string concat = code + b;
   // The output above gives me the correct concatenation.

   //However, I read some lines from the file.
   ifstream infile;

   string f3 = "temp.txt";
   string r;
   string temp;

   infile.open (f3.c_str ());

   while (getline (infile, r)) {
      temp = r + code;
      cout << temp << endl;
   }
   // The above would give me the wrong concatenation.
   infile.close();
   return 0;
}

it compiles and runs flawlessly. What does this do on your computer? If it fails, we may have to compare the contents of our temp.txt.

(This ought to be a comment rather than an answer, but it's too long. Sorry.)